1. Field of the Invention
The present invention relates to a location tracking method and a location tracking apparatus of a mobile communication system. More particularly the present invention relates to a location tracking method and a location tracking apparatus using an integrated Global Positioning System (GPS) and Time Difference Of Arrival (TDOA) scheme.
2. Description of the Related Art
As the modem society develops, personal portable communication is also rapidly advancing, and it is required to support a location based service (LBS) to a user equipment (UE) all over the world in order to provide emergency services such as an E-911 (emergency call) service. In particular, for the sake of the E-911 service, the United States of America prescribes the LBS as a basic function of the UE and recommends that the LBS be applied to all the UEs in future. Further, it is prescribed that the LBS of the UE should have an error within a range of 100 to 300 meters from an actual location so as to provide a comparatively precise location. Through the comparatively precise location of the UE, it is possible to provide various additional services related to location information.
FIG. 1 illustrates a mobile communication network architecture.
Referring to FIG. 1, the mobile communication network includes a plurality of base stations (BS) 21, 22, 23, 24 for providing mobile communication services to a mobile subscriber through a UE 10, a BS controller 30 for controlling the plurality of BSs, and a mobile switching center (MSC) 50 for connecting the BS controller 30 to another BS or public network.
The MSC 50 can reduce a time required for calling the mobile subscriber by detecting in which cell the mobile subscriber is located. However, in case of emergency, for example, when a fire or an urgent patient occurs, it is required to more precisely track a location of the mobile subscriber. Such more precise tracking of the mobile subscriber located in an arbitrary cell is called a location service. This location service is provided by a location tracking unit 40 which may be included in the BS controller 30 or exist separately.
A method for detecting a location of a UE in a mobile communication network is divided into two schemes, that is, a scheme in which location determination is carried out in a UE, and a scheme in which location determination is carried out in a mobile communication network.
A scheme using a GPS signal, which is one type of the former scheme, provides an precise absolute location of a UE by using satellite signals provided from a GPS satellite, so it has a tendency to be more increasingly used. However, although the GPS scheme provides an absolute location and high precision, it has a drawback in that signals are blocked in many areas.
In the latter scheme, at least three BSs transmit specific signals to a UE, and ranges between the respective BSs and the UE are obtained by using times of arrival, when the UE receives the signals, and times of departure, when the BSs transmit the signals. Next, based on the obtained ranges, a location of the UE is calculated using trigonometry. This scheme includes a Time Of Arrival (TOA) scheme and a TDOA scheme.
The TOA scheme uses ranges between BSs and a UE, which are calculated based on times taken to propagate specific signals from the UE to the BSs. That is, the UE is determined as located at an intersecting point of three circles, radiuses of which correspond to ranges between at least three BSs (21 to 23 in FIG. 1) and the UE, respectively. The TDOA scheme uses a set of points where time differences of arrival of signals, which at least three BSs receive from the UE, are constant, that is, a hyperbola. The UE is determined as located at an intersecting point of at least three hyperbolas. In comparison with the TOA scheme, the TDOA scheme is more complex, because the respective BSs must be exactly synchronized with each other. These TOA and TDOA schemes have a problem in that, since they use specific signals transmitted from the UE for location tracking, the transmitted signals may be delayed due to a multi-path, fading and so forth, and thus an actual location cannot be ensured.
Therefore, in order to solve this problem, a hybrids scheme, which simultaneously uses the GPS scheme providing an absolute location and the network scheme (TOA or TDOA) providing a continuous location, becomes necessary.